Apparatus for sealing plastics



K. F. SPALDING ET AL APPARATUS 'FOR SEALING PLASTICS Jan. 8, 1952 2SHEETS-SHEET 1 Filed Sept. 9, 1948 1952 K. F. SPALDING ET AL 2,581,977

APPARATUS FOR SEALING PLASTICS Filed Sept. 9, 1948 2 SHEETSSHEET 2Patented Jan. 8, 1952 APPARATUS FOR SEALING PLASTICS Kenneth F. spewinChicago, and Fred v.oo1 lins, Des Plaines, Ill., assignors to William F.

Stahl, Kenilworth, Ill.

Application September 9, 1948, Serial No. 48,458

This invention relates to sealing plastics; in particular it concerns anovel methodand apparatus by which a superior sealing action is obtainedby the application of heat to plastic surfaces after they have beenadjoined under compression in the desired position.

Prior-art plastic sealing techniques have, in general, employed one oftwo methods of achiev ing a sealing or bond between a pair of plasticsurfaces. In one of these methods. a pre-heated sealing element, such asa hot roller or a pair of hot sealing bars, has been employed, theplastic surfaces to be bonded being adjoined and subjected tocompression by the hot roller or between the hot sealingrbars. The othermethod known to the prior art has involved bringing the plastic sheetstogether and then subjecting them to a powerful radio-frequency electricfield, the dielectric losses thereby produced in the plastic beingeffective to heat the plastic to fusing temperature. 7

While it is more expensive, the radio-frequency method of sealingplastics has been much more successful with respect to a large number-.of plastic materials in general use, including polystyrene,polyethylene, and the various members of the polyvinyl family. Themethods .of the prior art, apart from the radio-frequency technique,have never been effective to achieve. a strong, stable bond in suchmaterials. When hot-bar or hot-roller sealing has been attempted, it hasbeen found that the portion of the" plastic material immediatelyadjacent the pre-heated bars or rollers has been raisedto an excessivetemperature, while the surfaces to be bonded together have failed tofuse.

We have discovered, and in this invention do provide, a method and meanswhereby plastic materials can be rapidly and effectively sealedtogether, with a bond as strong and, stable as that produced .byradio-frequency sealing, with: out the necessity for employingradio-frequency energy.

We have discovered that if a pair'of plastic sheets are adjoined inposition for sealing, placed under compression by mechanical memberswhich are substantially cooler than the fusing temperature of theplastic, .andthencaused to risedn temperature by the application ofheat, the temperature of the plastic under compression rises throughoutat substantially the same rateQan excellent seal is producedbetween theadjoined surfaces, and no running or other, undesirable effects ofexcessive temperature HOQCHiflQl'i :the

8 Claims. (01. 219 19) outer surfaces of the plastic under compress1on.

' We have discovered, moreover, that the failure of prior-art hot-presssealing techniques has been in greatdegree because theexistence of alarge temperature difierential between the inner and outer surfaces ofplastic sheets adjoined for sealing inhibits the molecular reorientationwhich must occur within the plastic material in order to achieve a goodbond.

In our invention, which might be described as a cold-press sealingmachine, the plastic sheets to be sealed are held under'compression bysealing bars which are initially at a temperature well'below themeltingv point of the plastic. The bars and the plastic are then heated,together simultaneously, and the net temperature differential betweenthe coolest-portion of the-plastic and the hottest portion is verysmall. a

In the embodiments of our invention hereinafter described, the sealingbars which compress the plastics to be sealed areprovided withelectrical heating elements which may be operated from conventionaldirect or alternating current sources. Switch means are provided forapplying current to the electrical heating elements after the bars haveengaged the plastic sheets and for turning off the current, to permitthe bars to cool, after the seal has been accomplished.

In one of the embodiments of the invention hereinafter described,,wehave provided sensitive, automatic means for turning off the currentthrough the electrical heating elements at the instant that the optimumtemperature for sealing has been reached.

An object of this invention is to provide a method and means foreffectively sealing. together sheets of thermoplastic materialwithoutthene cessity for employing radio-frequencyenergy.

Another object of this invention is to. provide apparatus wherein sheetsof thermoplastic material are adjoined under compression betweenrelatively cool. mechanical members and then sealed together by raisingsimultaneously the surface temperature of such mechanical members andthe plastic sheets secured therebetween.

Still another object of this invention is to provide a plastic sealingmachinewhich has auto-,- matic temperature control, adjustable to permituse of the sealing temperature best adapted for the particular plasticbeing treated. e A stil1 further object of thisinvention is to pro videa cold-press plastic sealing machine wherein sheets of thermoplasticmaterial are com pressed between relatively cool sealing bars which arethereafter raised in temperature to seal the plastic sheets together andautomatically cooled after the seal is effected.

Our invention is illustrated, in two embodiments, by the accompanyingdrawings, of which Figure 1 is a side view, in elevation, of aplasticsealing machine designed for making-a straightline seal such asmight be useful in providing closure for plastic envelopes or otherplastic containers; Fig. 2 is a fragmentary view, in front elevation, ofthe sealing bars employed in the machine of Fig. 1; Fig. 3 is aschematic showing of another embodiment of our invention wherein theheat supplied to the sealing bars of the machine can be turned offautomatically when the optimum temperature for sealing has been reached;Fig. 4 is a perspective view, partly in section, of a pair of sealingbars constructed for use in the embodiment of the invention shown inFig. 3; and Fig. 5 is a view in cross section of a pair of sealing barsshowing how a pair of plastic sheets may be adjoinedbetween the bars andheld therein in position for heat-sealing.

In Fig. l, the operative components of the plastic-sealing machine aresupported on a frame II] which comprises a horizontal, tablelike portionIlla and a secondary upper portion IIlb which rises from the rear of themiddle portion of table Illa and is bent forward to overhangtable-portion I001. as shown. A sealing bar I2 is carried laterallybytable Ito and is secured thereto by any suitable means (not shown).The upper surface of sealing bar I2 contains a recess within which isfitted a strip of insulating material I3; set into strip I3 is anelectrical heating element l4, consisting of a strip of material havingthe properties of substantial electrical resistance and ability towithstand high temperatures. Many materials are well known and generallyavailable for forming such heating elements. Sealing bar I2, insulatinginsert I3, and heating element I4 are allmachined to provide a smooth,continuous upper surface. The second sealing bar I5 is positioneddirectly above sealing bar I2, and the lower surface of sealing bar I5carries an insulating insert I6 and a heating element I'i located soasto register with the equivalent elements I3 and I4 in sealing bar I2.Upper sealing bar I5 is attached, by a pair of stand-off supports I8, toa lateral supporting'bar I9 a. At its cene tral point, bar I9a issecurely affixed to a piston rod I9 which rises vertically from bar I9a,through a suitable aperture in frame extension Iflb, to enter pneumaticcylinder 20, which is positioned above and secured to frame extensionIllb'.

The upper end of piston rod I9 is joined to piston a, which isreciprocable within pneumatic cylinder 20.

Apipe 2| connects the upper end of cylinder 20 to port 23c on one sideof control valve 23; a pipe 22 connects the lower end of cylinder 22 toport 2311 on the opposite side of valve 23. A pipe is provided toconnect port 23c in valve 23 to any convenient source of pneumaticpressure, while port 23a in valve 23 is open to atmosphere. A rotaryvalve member 23b, movable within valve 23, contains ports operative, asmay be seen from Fig. .1, alternately to connect pipe 25 to pipe 22,while venting pipe 2I, or to connect pipe 25 to pipe 2|, while ventingpipe 22, according to whether movable member 23b is in the positionshown in Fig. 1 or in a position 90 clockwise thereto. able valve member2317; an arm 26 is pivotally A lever 24 is rigidly connected tomovconnected to the free end of lever 24. As shown. arm 26 extendsdownward to a point below the table Illa, where it may be connected to afoot pedal for convenient actuation by the operator of the machine.

As shown in Fig. 2, the ends of electrical resistance element I! areconnected to the corresponding ends of electrical resistance element I4by a pair of flexible connecting cables 35. 'Cable 32 runs from one endof electrical heating element I4 to one terminal of a micro-switch 30.The other-terminal of micro-switch 30 is connected to flexibleelectrical cable 33. Electrical cable 34 is connected to the end ofelectrical heating element I4 opposite micro-switch 30. Cables 33 and 34may be connected to any suitable source of current, such .as aconventional volt power line.

Micro-switch 30 is mounted on table Illa near one end of sealing bar I2.A projecting finger like member 3I is mounted on sealing bar I5,extending forward and downward so as to engage the arm of micro-switch30 when sealing bar I5 is resting on sealing bar I2. Micro-switch 3B isnormally open, and is closed when its arm is engaged by member 3I.

In the operation of the plastic-sealing machine shown in Figs. 1 and 2,the operator will normally place the plastic sheets to be joinedtogether, such as the open end of an envelope or other container, on theupper surface of sealing bar l2. The operator will then, by actuatingarm 26, cause piston 20a to drop, thereby bringing sealing bar I5 downonto sealing bar I2 and compressing the plastic material tightly betweenthe two sealing bars.

As sealing bar I5 drops, member 3| engages the arm of switch 30, closingthe switch and causing electrical current to flow through heatingelements I4 and I1. As the temperature of elements I4 and I! rises, thetemperature of the plastic compressed therebetween rises accordingly,and when the plastic reaches the temperature of fusion, the two plasticsheets are bonded together into a single homogeneous layer. Normally thetime required to effect a bond is from one to three seconds. Whensufiicient time has elapsed for the bond to be formed, the operatorshifts arm 26 to the position shown in Fig. 1, sealing bar I5 rises, andthe current through heating'elements I4 and I1 is shut off.

Since the mass of the heating elements I4 and I! 'is'very small, andsince insulating inserts I3 and I6 prevent the transmission of anysubstantial quantity of heat to the bars I2 and I5, the sealing bars,including the heating elements I4 and I1, cool off to a temperature wellbelow the melting point of the plastic within a fraction of a secondafter the current is cut off. It is therefore possible for the operatorto keep the machine in substantially continuous operation, making a newseal every few seconds, and at the same time have each new pair ofplastic sheets compressed between relatively cool members before theyare heated.

Figs. 3 and 4 show an embodiment of our invention in which automaticmeans are provided for'turning off the current through the heatingelements carried by the sealing bars when they have reached the optimumtemperature for sealing. The automatic feature of the embodiment ofFigs. 3 and 4 assures that perfect seals will be obtained with thoseplastic materials which are highly critical as to sealing temperature,and, in addition, it relieves the operator from the responsibility fordetermining how long the plastic should be subjected to heat andpressure for proper sealing.

Fig. 3 being schematic, a pair of sealing bars H and II I are thereinshown diagrammatically. These bars may be in general similar to thesealing bars I2 and I of the Fig. 1 embodiment, although, as indicatedin Fig. 4, it is preferable that the main body of the bars be formed ofsome insulating material, such as porcelain or other ceramic. sealingbars are shown in Figs, 3 and 4 as being adapted to form a straight-lineseal. It will be understood that with respect to both the Fig. 1 andFig. 3 embodiments, this choice as to the shape of the sealing bars ispurely byway of example, since the bars may take any-form ap-''propriate to the task at hand.

As shown best in Fig. 4, sealing bars-I I0*and II I have flat lower andupper faces respectively which are adapted to register with one another.Bar IIO has in its lower face a shallow longitudinal recess whichcarries a heating element II5, machined so as to fit flush with the faceof bar IIO. A similar recess in the upper face of sealing bar IIIcarries -a heating element II6. Heating elements H5 and H6 may besecured to the respective sealing bars I I0 and III by any suitablemeans, such as flat head screws III, threaded through apertures suitablycountersunk to preserve a smooth surface on the cooperating inner facesof sealing bars I I 0 and I I I. I

Figs. 3 and 4 do not show anymechanical structure for mounting sealingbars I I 0- and ;I I I; any suitable arrangement; such, for example, asthat used in the Fig. 1 embodiment, may be employed to carry the barsand to provide for their being brought together and moved apart at theoperators will. It will be understood that whatever type of mechanicalmounting is used should provide for registration of heating elements H5and I I6 when the bars are brought together.

As shown in Fig. 4, sealing bar III contains a vertical bore I I9positioned to intersect the recess carrying resistance heating element II6. Bore H9 is internally threaded near its lower end to receivethreaded bushing I23. Within bore I I9 a sensitive thermocouple unit I20is carried, held in place by bushing I23, as indicated in Fig. 4. Thelead wires I2I and I22, extending from thermocouple I20, pass out of barIII through the central aperture in bushing I23.

As shown schematically in Fig. 3, resistance- I51 of a power transformerI55. The leads I2I' and I22 of thermocouple I20 are connectedrespectively to the opposite corners of a Wheat-" stone bridgecomprising fixed resistors I25, I25, and I28, and variable resistorI2'I. connected to the junction of resistors I25 and I26, while lead I22is connected to the junction of resistors I 2? and I 28. The junction ofresistors I25 and I2'I is connected to one end of coil I3I of asensitivemoving-coil relay I30. The other terminal of coil I 3I is' connected to'the junction of resistors I26 and I28.

Relay I 30 should be' a direct-current instrument responsive to verysmall currents; normally it will be designed in a, manner similar to aAs in the Fig. 1 embodiment, the

dArsonval galvanometer--that is, coil I3I would be a delicately-balancedmoving coil carrying a contact positioned to engage a fixed contact whencoil I 3| attains a critical deflection. In the drawing, relay I30 isrepresented schematically in the usual form; the moving contact isdenoted I34 and the stationary contact has the reference numeral I33. Asindicated, relay I30'has normally-open contacts, which close uponappropriate actuation of coil I3 I.

Contact I34 is connected to one terminal of coil MI in relay I40. Theother terminal of coil MI is connected to one side of a current sourceI35, represented schematically in the drawing as a battery. The otherterminal of current source I35 is connected to contact I33 on relay I30.Relay I40 may be of the conventional type having a stationary coil I4I,an armature I43, and a pair of normally-closed contacts I44 and I45. Itwill be understood that While current source I35 is shown in Fig. 3 as abattery, it may be any convenient source of either direct or alternatingcurrent adapted to actuate the coil I4I of relay I40. 4

Relay I50 is a conventional relay having a stationary coil I5I, anarmature I52, and a pair of normally-open contacts I53 and I54. In theembodiment shown in the drawing, a main current source (designated LINE)is shown; the main current source may be the usual commercial lineproviding alternating current at or 230 volts. Relay I50 should have acoil I5I de-; signed for operation on alternating current of theparticular voltage and frequency provided by the line.

'One terminal of coil I5I is connected to contact I45 of relay I40; theother terminal of coil I5I is connected to line bus I6I. Contact I44 onrelay I40 is connected to one terminal of a normally-open push-buttonswitch I60; the other terminal of switch I60 is connected to line busI62. One terminal of primary coil I56 of power transformer I55 isconnected to line bus I6I; the other terminal of coil I56is connected tocontact I44 on relay I40. Contact I53 on relay I50 is connected tocontact I4 of relay I 40; contact I54 on relay I 50 is connected to linebus I52.

In the operation of thisembodiment of the invention, after a pair ofplastic sheets have been lapped in the desired position and securedbetween sealing bars H0 and I II, pushbutton switch I60 may be manuallyactuated. (Fig. 5 shows, in cross section, a pair of plastic sheets II 2and H3 as they might be placed between flow from line bus IBI throughcoil I 5|, closed,

contacts. I44 and I45, and switch I60 back to line I62. This will causecontacts I53 and I54 toclose, and thereuponcurrent will continue to flowthrough transformer winding I56 and relay coil I5I even thoughpush-button switch IE0 is released. Thus theapparatus is set into actionby a momentary closing of switch I60. 7

The currents induced in secondary coil I51 of transformer I55 flowthrough heating elements II5 and H6, causing their temperature to riseand thus heating the plastic sheets H2 and H3 lapped between the sealingbars I I 0 and III. As current continues to flow through heating elementI I6, thermocouple I20, positioned adjacent thereto, grows hot, and aminute potential is a c cordingly developed .acrossits terminals. AportlOIl of this potential, depending on the setting of resistor 21,isapplied to coil I31 of sensitive relay I30, and; accordingly a smallcurrent is caused to flow in coil I31. As the temperature ofthermocouple I29 increases, the current through coil I3I- growsprogressively greater until, at some critical temperature, contacts I33and I34 are caused to close. relay I 40 is at once actuated and contactsI44 and M open. Thereupon current ceases to flow through coil I5I,contacts I53 and I54 of relay i'50 are opened; and current ceases toflow through the heating elements I and INS. The sequence of: relayoperations 'just described occurs substantially instantaneously;accordingly the plastic sheetssecured between bars IIIl-and III at oncebegin to cool; and in a second or less will normally re-assume theirsolid state, bonded into a single piece.

As soon as the temperature of thermocouple and substituted a new pair ofsheets between bars IIO and'III.

The critical temperature at which the relays operate to cut off thecurrent can be precisely controlled by adjustment of resistor I21.Should resistor I21 be adjusted to that setting which balances perfectlythe Wheatstone bridge, no potential willappear. across coil. I3!regardless of the temperature of thermocouple I20. As resistor I21 isvaried from that point which balances the bridge, an increasingly largerpercentage of the totalpotential developed by thermocouple I isimpressed across coil I3I. To place this invention in operation,therefore, that setting of resistor I21 is experimentally determinedwhich gives relay operation at the optimum sealing temperature of theparticular plastic in use. If the machine is to be used at various timeson a variety of diiferent types of plastic material, the settingv ofresistor I21 appropriate to each type of plastic can be determined andthe knob or dial which controls resistor I21 can be directly marked orotherwise calibrated, so that when a new material is to be sealed by themachine, re-

sistor I21v may at once be shifted to the proper setting. It will beunderstood that a simple resistance voltage-divider network may be usedto control the critical temperature, if desired, in lieu of a Wheatstonebridge.

Whilewe have shown, in the embodiment of the invention illustrated inFig. 3, a manuallyoperable push-button switch as the means af forded forinitiating. a sealing operation, many variations of thisarrangement maybe made according to the requirements of the task to be perto be sealed,the heating element on the other.

sealing bar may be dispensed-with. It has been When this occurs,

found that the use of a. single heating element same, and relay meansoperative responsively to the control element to cut oficurrent to theheating element when said heating element reaches a predeterminedtemperature.

2. In apparatus for sealing. together adjoined plastic surfaces, meansfor holding plastic material in predetermined position for sealing, aheating element adapted to raise the temperature of such plasticmaterial, a thermocouple positioned adjacent said heating element andresponsive to heat from the same, and relay means operative responsivelyto the thermocouple to cut off electric current to the heating elementwhen the temperature of the thermocouple rises to a predetermined level.

3. In apparatus for sealing together adjoined plastic surfaces, a pairof sealing bars adapted to receive and hold plastic material inpredetermined position for sealing, an electrical heating elementcarried by one of said bars adapted when carrying current to heat saidplastic material, a thermocouple positioned adjacent said'heatingelement and responsive to heat lfrom the same, and relay means operativeresponsively to the thermocouple to cut of]? electric current to theheating element when the temperature of the thermocouple rises to apredetermined level.

4. In apparatus for sealing plastics, a pair of sealingbars adapted toreceive and hold plastic material in predetermined position for sealing,an electrical heating element carried by one of said bars adapted whencarrying current to heat said' plastic material, a thermocouplepositioned adjacent said heating element and responsive to heat" fromthe same, relay means operative upon actuation to cut oii current fromthe heating element, and a network comprising a variable resistorcoupling the thermocouple to the relay means operative to actuate therelay means when the thermocouple reaches a critical temperaturedetermined by the adjustment of the variable resistor.

5. In apparatus for sealing plastics, a pair of sealing bars adapted toreceive and hold plastic material in predetermined position for sealing,an electrical heating element carried by one of said bars adapted whencarrying current to heat said plastic material, a thermocouplepositioned adjacent said heating element and responsive to heat from thesame, relay means operative upon actuation to cut off current from theheating element, and a Wheatstone bridge comprising a variable resistorcoupling the thermocouple to the relay means operative to actuate therelay means when the temperature of the thermocouple reaches a criticallevel controlled by the adjustment of the variable resistor.

6. In apparatus for sealing plastics, a pair of sealing bars adapted toreceive and hold in predetermined position plastic material for sealing,an electrical heating element carried by one of said bars adapted whencarrying current to heat said plastic material, a thermocouplepositioned adjacent said heating element and responsive to heat from thesame, control means for the heating element comprising a relay and aswitch operative upon actuation of the switch to feed current to theheating element and upon actuation of the relay to cut off current tothe heating element, and means coupling the thermocouple to the relayoperative to actuate the relay when the thermocouple reaches apredetermined temperature.

7. In apparatus for sealing plastics, a pair of sealing bars adapted toreceive and hold in predetermined position plastic material for sealing,an electrical heating element carried by one of said bars adapted whencarrying current to heat said plastic material, a thermocouplepositioned adjacent said heating element and responsive to heat from thesame, control means for the heating element comprising a relay and aswitch operative upon actuation of the switch to feed current to theheating element and upon actuation of the relay to cut oil current tothe heating element, and an electrical network comprising a variableresistor coupling the thermocouple to the relay operative to actuate therelay when the thermocouple attains a critical temperature determined byadjustment of the variable resistor.

8. In apparatus for sealing plastics, a pair of sealing bars adapted toreceive and hold plastic material in predetermined position for sealing,an electrical heating element carried by each of said bars adapted whencarrying current to heat said plastic material, a thermocouplepositioned adjacent one of said heating elements and responsive to heatfrom the same, and relay means operative responsively to thethermocouple to cut ofi current to the heating elements when thethermocouple attains a predetermined temperature.

KENNETH F. SPALDING.

FRED V. COLLINS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,093,745 Westell Sept. 21, 19372,289,618 Young July 14, 1942 2,325,094 Ashworth July 27, 1943 2,360,277Robertson Oct. 10, 1944 2,369,362 Marziani Feb. 13, 1945 2,402,298Salfisberg June 18, 1946 2,422,734 Jung June 24, 1947 2,460,460 LangerFeb. 1, 1949 2,488,422 Mershon Nov. 15, 1949

